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FIGURE 13.9 Trend in column O, (a) from November 1984 to May 1991 based on SAGE II measurements in the stratosphere (at pressures above 82.5 mbar, • and dashed lines) and TOMS measurements (solid line) and (b) from January 1988 to May 1991, which was a period of relatively small and stable aerosol particle concentrations. Note change in vertical scale. (Adapted from Cunnold et al., 1996.)

Fig. 13.9a. Cunnold et al. (1996) indicate that these errors in accurately extracting the aerosol contribution were responsible for about half of the discrepancy and increases in tropospheric ozone the other half. Similar discrepancies between SAGE measurements and the results from other techniques such as ozonesondes have also been reported and attributed largely to the problem of accurately removing the aerosol particle contributions (e.g., Veiga et al., 1995; Steele and Turco, 1997b).

All of these approaches, as well as others such as the MLS (Microwave Limb Sounder) that have not been used for sufficient lengths of time to give long-term trends, have some uncertainties associated with them, as is the case for all analytical techniques (e.g., see Harris et al., 1997). For example, regular calibration is particularly difficult with satellite-based measurements and care must be taken to ensure that a continuous degradation in the instrument sensitivity, for example, is not interpreted as a trend in total ozone.

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